The present invention relates to a rotation detector, and more specifically, to a rotation detector including a rotor having magnetic sensing elements.
FIG. 10 illustrates how an absolute position detection type detector detects rotational position. The rotation detector includes a rotor 30, which is fixed to a rotating shaft and integrally rotates with the shaft. N pole zones 32 and S pole zones 33 are alternately formed at sixty degree intervals on the rotor 30 in a circumferential direction. At positions facing the rotor 30, first to third magnetic resistance elements 31 are arranged around the axis O of the rotor 30 at forty-degree intervals. Each of the first to third resistance elements 31 detects the N pole zone 32 and the S pole zone 33, which alternately pass by the resistance elements 31 at sixty-degree intervals during the rotation of the rotor 30.
When the N pole zone 32 is detected, the first to third resistance elements 31 respectively output H-level signals SG1, SG2, SG3. When the S pole zone 33 is detected, the resistance elements 31 respectively output L-level signals SG1, SG2, SG3. When each resistance element 31 detects a change from the N pole zone 32 to the S pole zone 33, each of the signals SG1, SG2, SG3 changes from the H level to the L level. Contrarily, when each resistance element 31 detects a change from the S pole zone 33 to the N pole zone 32, each of the signals SG1, SG2, SG3 changes from the L level to the H level.
As shown in FIG. 10, the signals SG1, SG2, SG3 of the resistance elements 31 change gradually between the L and H levels. The reason for this is that the direction of magnetic flux changes gradually when the detected zone changes from the N pole zone 32 to the S pole zone 33. Three comparators (not shown) respectively receive the signals SG1, SG2, SG3 and adjust the waveforms of the signals SG1, SG2, SG3, thus generating detection signals S1, S2, S3 that change sharply between the L and H levels.
More specifically, each comparator compares an output signal with a reference value, which is a middle level between the H level and the L level. When the output signal is greater than the reference value, the comparator generates an H level detection signal S1, S2, S3. When the output signal is lower than the reference value, the comparator generates an L level detection signal S1, S2, S3. The reference value is the level of the signals SG1, SG2, SG3 output when the border between the N pole zone 32 and the S pole zone 33 passes by each of the first to third resistance elements 31. When any one of the detection signals S1, S2, S3 changes from the L level to the H level or from the H level to the L level, the rotational position of the rotor 30 (or rotation shaft) is determined based on the state of the other detection signals. In the case of FIG. 10, the rotational position (absolute position) is detected in the range of zero to hundred twenty degrees at intervals of twenty degrees.
However, it is difficult to precisely form the N pole zone 32 and the S pole zone 33 alternately at sixty-degree intervals on the rotor 30 in the circumferential direction. Accordingly, the rotational position is not detected at twenty-degree intervals with precision at the point when the detection signals S1, S2, S3 change from the L level to the H level or from the H level to the L level.
The objective of the present invention is to provide a rotation detector that detects rotational position with high precision.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a rotation detector is provided. The rotation detector includes a rotor having a shaft, magnets and magnetism detection elements. Axial projections are located at predetermined intervals about the rotor. The magnets are located between the projections and the shaft. Each magnetism detection element detects magnetism of corresponding one of the magnets and includes a plurality of magnetic resistors. A movement of the projections changes the flux of the magnets. Each resistor is switched between a first state, in which its resistance is great, and a second state, in which its resistance is small. The resistors are divided into two groups. The groups are substantially symmetrical relative to a group centerline such that, when the direction of the flux changes, the resistors in one group are in the opposite state from that of the resistors in the other group. The centerline of changes in the direction of the flux caused by movements of the projections matches the group centerline.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.